So...I asked Nelson Pass (duh!). He says that, yes, it is 11K single ended. He added that I could insert
resistance in series with the input if I like. The gain of the X150 is about 30 dB, so if I put 22K in series I would still get 20 dB gain.
What do people think of this idea?
By the way, the reduction in gain should not be a problem, at least on the face of it. The lowest setting of the First Sound is sometimes too loud as it is.